Increasing utilization of mobile broadband technology worldwide results in significantly increasing traffic volumes that need to be handled by the mobile or wireless communication networks (e.g. WCDMA/HSPA). Therefore, techniques that allow network operators to manage their network more efficiently are of large importance. Some approaches allowing, e.g., to improve the downlink performance, would be to introduce support for 4-branch MIMO, multiflow communication, multi carrier deployment etc. Since the spectral efficiency per link is approaching theoretical limits, it may be considered to improve the spectral efficiency per unit area. It is desirable that additional features for mobile networks, e.g. via networks based on HSDPA or LTE, provide a uniform user experience anywhere inside a cell, which may require changing the topology of traditional networks. For example, heterogeneous network deployments may be considered in this context.
Deployment of low-power nodes (LPNs) to form a heterogeneous network may be seen as a tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit (RRU), pico, femto or micro base station, allowing expanding the network capacity in a cost-efficient way. Note that the power transmitted by these LPNs may relatively small compared to that of macro base stations, e.g. 2 W as compared 20 W for a typical macro base station.
In the context of this description, a network comprising macro Nodes like eNodeBs and/or LPNs, and/or, more generally, nodes of differing transmitting power, which may serve at least partly the same cells and/or user equipments, may be referred to as a heterogeneous network. Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localised traffic hotspots. It may be considered a network as heterogeneous network if the user equipment identifies it and/or it appears to a user equipment as heterogeneous in regards to the relative transmitting power of the nodes of the network to the user equipment, which the user equipment may receive at different power levels. Generally, the transmitting power referred to herein may be a nominal and/or actual and/or maximum transmitting power of a node or terminal and/or in a cell. A user equipment may generally perceive a network with multiple nodes communicating with the user equipment as a heterogeneous network, e.g. if the power received from different nodes is different, in particular significantly different, e.g. due to the actual transmitted power of the nodes (e.g. due to transmitting power control) differing and/or due to differences in signal paths and/or distance between the user equipment and the node.
Deployed nodes like LPNs in a heterogeneous network are typically classified as either co-channel (or named as other terms e.g., separated cell), where each LPN has its own cell, which may have its own cell identity (e.g., primary scrambling code in UMTS network), or combined cell (or named as other term, e.g., soft cell) where the LPNs provide cells with the same cell identities as a Macro cell they are associated to.
FIG. 1 shows an example of co-channel deployment of a Macro station and an LPN. Employing low-power nodes in a macro cell in a co-channel fashion offers load balancing (by enabling traffic offloading to LPNs), which may bring large capacity gain, both regarding average system throughput and regarding cell edge user throughput.
In a heterogeneous network, the DL transmission power of the one or more of the nodes or cells, in particular that of one or more macro nodes or cells, may be significantly higher than that of other node or cells, in particular of LPNs. This may create an imbalance region where the power received by a user equipment or other entity via a DL from a first node, e.g. a macro node, it is in wireless communication with is higher than the power received by the same user equipment via a, possibly simultaneous, downlink connection from a second node, e.g. an LPN or another node, e.g. a more distant macro node, but the uplink connection to the second node like the LPN or correspondingly, the other node, is better, in particular in terms of power received by the respective node from the user equipment or other entity, than the uplink connection to the macro node, e.g. in cases the user equipment is closer in distance to the LPN than to the macro node. The other entity may be a mobile or non-mobile entity or node capable of and/or in wireless communication with one or more nodes of the wireless communication network. The other entity may for example be a network node, in particular a LPN, and/or a relay node and/or access point and/or a micro node and/or a pico node and/or a femto node.
If the user equipment or other entity is in an imbalance region, but not in a SHO (SHO=Soft HandOver) area, the UE is power controlled by first node, e.g. a Macro node or cell, such that the first node or Macro node or cell determines the uplink transmission power of the user equipment or other entity. In this case, the UE uplink transmission to the first node and/or in the corresponding cell, e.g. the Macro cell, may create high interference to the second node, e.g. the LPN node, which may be much closer physically to the user equipment or other entity than the controlling first or Macro node.
If the UE is in SHO, its UL power control may be handled jointly by the first and second node, e.g. a macro node and a LPN. However, due to the much better UL to the LPN, it will effectively be controlled by the LPN. If the first node or macro node is or provides the serving cell, the power received by the first node from the UE or other entity via the UL connection may lower than desired or useful. This may lead to inadequate reception via an UL control channel, e.g. via HS-DPCCH (High Speed Dedicated Physical Control Channel, which carries the DL channel quality indication (CQI), and HARQ (Hybrid Automatic Repeat Request) acknowledgement), which in turn may lead to inefficient or ineffective communication via a corresponding DL channel, e.g. via HS-DSCH (High Speed Downlink Shared Channel). Another issue has been observed is that E-DPCCH (E-DCH Dedicated Physical Control Channel) may suffer from the reduced reliable transmission of the happy bit, which in turn leads to the degraded EUL (Enhanced Uplink) data rate.
In cases where the LPN offload is increased by small cell range expansion, an even higher imbalance may create high interference for UEs served by the LPN, which may lead to reduced DL throughput. FIG. 2 illustrates an example of a corresponding imbalance region.
Several solutions to combat the negative effects of the UL/DL imbalance are known. These methods for example include power boosting of UL control channels, inner-loop power control restrictions and introduction of a secondary pilot. These methods are applicable for legacy UEs and to some extent create additional UL overhead from the added or enhanced control channels. Other methods such as E-DCH (Enhanced Dedicated Channel) decoupling can improve the reception of or via the E-DPCCH (E-DCH Dedicated Physical Control Channel), hence may improve the E-DCH transmission by directly serving a UE via LPN. However, it doesn't fix the issue of the robustness of UL control channel reception at the Macro cell. This means that E-DCH decoupling does not solve the imbalance issues for the control channels, it merely enhances the E-DCH transmissions by moving the UL scheduling to the LPN instead of the macro node.
Another alternative which to some extent aims to limit the UL/DL imbalance is to use a DF-DC (Dual Frequency-Dual Cell) approach with reduced macro power on the second carrier. This is a rather special case which involves two carriers. However, the DF-DC does not give an offloading gain compared to DC operation, and there is an additional cost of doubled signaling overhead for DF-DC due to that both carriers may have to perform measurements and the serving cell change independently. Furthermore, reducing transmission power on the second carrier may reduce performance for legacy UE served on the carrier subject to power reductions.